1. Filed of the Invention
The present invention relates to an optical compensation polarizing film and a liquid crystal display (LCD) device, and more specifically to an optical compensation polarizing film capable of improving viewing angle characteristics concerning contrast ratio and chromaticity, and an LCD device of a homogeneous orientation type using such an optical compensation polarizing film.
2. Description of the Related Art
There has been a proposal for an IPS (in-plane switching)-mode LCD device in which an electric field parallel to a substrate is generated between a pixel electrode and a common electrode. The IPS-mode LCD device is known for its capability of allowing a wider viewing angle than a TN-mode LCD device. The IPS-mode LCD device generally has a pair of substrates, a liquid crystal layer sandwiched between the paired substrates, and first and second polarizing layers respectively adhered onto the outer surface of the substrates. In the LCD device, orientation directions of the polarizing films and the liquid crystal layer at initial orientation are such that black color is displayed when no electric field is applied between the electrodes. The orientation direction of the liquid crystal layer is changed so as to obtain an optical path of λ/4 when an electric field is applied, thereby displaying white color.
In the LCD device, polarizing films are used to control transmission of light by changes in orientation of liquid crystal. The polarizing films each are usually constructed by layering a transparent protection film (TAC: triacetylcellulose) on a polarizing layer. The polarizing layer is an optical element which divides incident light into two polarization components perpendicular to each other. One of the components that has an oscillation direction parallel to the transmission axis of the polarizing layer is allowed to pass through the LCD device. The other one of the components that has an oscillation direction parallel to the absorption axis of the polarizing layer is absorbed or dispersed.
LCD devices having a homogeneously-oriented liquid crystal layer, including a transmissive IPS type, are constructed by sandwiching liquid crystal cells between polarizing films from both sides in the thickness direction of the cells. In general, polarizing layers in both sides are positioned such that transmissive axes are perpendicular to each other. A pair of polarizing layers in which transmission axes thereof are arranged perpendicular to each other are called an orthogonal polarizer.
Generally, characteristics of an orthogonal polarizer depend on viewing angles. When light enters from a direction oblique to the orthogonal polarizer, directions of transmission axes change. Therefore, even if first and second polarizing layers are layered such that transmission axes of these layers are perpendicular to vertically incident light, crossing angles with respect to obliquely incident light deviate from right angles. As a result, polarized light which has passed through the first polarizing layer has a component in the direction parallel to the transmission axis of the second polarizing layer. This component transmits through the second polarizing layer, causing leakage of light.
Dependency of the orthogonal polarizer on viewing angles, as described above, adversely affects luminance and contrast ratio of the screen of the LCD device and becomes a factor which narrows a visible range (viewing angle) in which a suitable chromaticity can be achieved. To realize an LCD device having a wider viewing angle, developments of an optical compensation polarizing film as described below are necessary. That is, an optical compensation polarizing film has to be developed to suppress dependency of the orthogonal polarizer on viewing angles, to thereby increase the visible range (viewing angle) in which leakage of light is not perceived. Several proposals have been made for such an optical compensation polarizing film.
JP-A-2001-242462 describes a technique to restrict leakage of light as described above in a homogeneously-oriented LCD device, such as an IPS-mode LCD device. In the described technique, a biaxial-birefringent film (optical compensation film) is used to achieve compensation such that a component of obliquely incident light that is parallel to the transmission axis of a second polarizer becomes perpendicular thereto, thereby suppressing leakage of light.